In an air conditioner disclosed in U.S. Pat. No. 5,526,650 (corresponding to JP-A-8-40056), when a dehumidifying and heating operation is performed, high pressure refrigerant discharged from a compressor is circulated into a first interior heat exchanger to heat air to be blown into a passenger compartment. Then, refrigerant flowing out of the first interior heat exchanger is decompressed, and the decompressed refrigerant is circulated into an exterior heat exchanger and a second interior heat exchanger in this order, so that air to be blown into the passenger compartment is cooled and dehumidified by the second interior heat exchanger. In this way, in the dehumidifying and heating operation, air is cooled lower than its dew-point temperature to be dehumidified, and the dehumidified air is heated.
However, if the dehumidifying and heating operation is performed when an outside air temperature is lower than 0° C., that is, when an air conditioning load (heating load) is large, the following problems occur. In this air conditioner, refrigerant flowing out of the exterior heat exchanger is decompressed, and the decompressed refrigerant is circulated into the second interior heat exchanger, so that the refrigerating capacity of the second interior heat exchanger can be obtained. At this time, a refrigerant temperature in the exterior heat exchanger is higher than that in the second interior heat exchanger. Further, a refrigerant pressure in the second interior heat exchanger is required to be controlled at a pressure corresponding to a temperature (e.g., 2–3° C.) where the second interior heat exchanger is not frosted. Thus, the refrigerant pressure in the exterior heat exchanger necessarily becomes higher than that in the second interior heat exchanger.
In a case where the outside air temperature is about 0° C., the refrigerant temperature in the exterior heat exchanger may become higher than the outside air temperature. In this case, the exterior heat exchanger is operated as a radiator. Accordingly, when the outside air temperature is low, a heat absorption amount in the exterior heat exchanger is reduced due to a decrease of an outside air temperature, and a heat absorbing operation cannot be performed in the exterior heat exchanger. As a result, air to be blown into a passenger compartment may be not sufficiently heated.